Photo-cationic polymerization of cyclo-aliphatic epoxy group has many advantages as compared with photo-radical polymerization of acryl group. Since the photo-cationic polymerization of cyclo-aliphatic epoxy group is not sensitive to oxygen and has living characteristics, it can produce a cured product having a higher curing degree, as compared with the photo-radical polymerization of acryl group, which is sensitive to oxygen and has no living characteristics. In addition, the inert gas atmosphere does not need to be maintained during curing, resulting in a more simple process. The cyclo-aliphatic epoxy cured product by the photo-cationic polymerization, that is capable of realizing a high curing degree, has many advantages, such as, low shrinkage, excellent mechanical and electrical properties, excellent chemical resistance, excellent adhesion with various substrates, and the like, as compared with an acryl cured product by the photo-radical polymerization. However, the photo-cationic polymerization of cyclo-aliphatic epoxy resin is less frequently employed in industries because of a lower photo-curing speed than the photo-radical polymerization and limited kinds of precursors. (Prog. Polym. Sci., 21, 593-650 (1996))
A photo-cationically polymerizable cyclo-aliphatic epoxy group-containing oligosiloxane resin synthesized by a sol-gel reaction has an advantage in that it is faster in photo-cationic polymerization than a photo-cationically polymerizable cyclo-aliphatic epoxy group-containing monomer/polymer, due to the presence of siloxane networks therein. In addition, due to the siloxane networks, a cured product of the cyclo-aliphatic epoxy oligosiloxane resin has low shrinkage, high thermal stability, excellent chemical resistance, and the like, as compared with a cured product of the cyclo-aliphatic epoxy group-containing polymer. However, the rate of photo-cationic, polymerization of the photo-cationically polymerizable cyclo-aliphatic epoxy oligosiloxane resin is still insufficient as compared with that of photo-radical polymerization of acryl monomer/polymer.
Therefore, the speed of photo-cationic polymerization needs to be improved in order to extend the application of the photo-cationically polymerizable cyclo-aliphatic epoxy oligosiloxane resin. Meanwhile, the cured product of the cyclo-aliphatic epoxy oligosiloxane resin, of which the rate of photo-cationic polymerization is increased, may have higher cross-linking density, and thus, it is anticipated that it has excellent mechanical properties, thermal-mechanical properties, and electrical properties, as compared with cured products of the existing cyclo-aliphatic epoxy oligosiloxane resin.